The present invention relates to an ultrasonic oscillating device and to an ultrasonic washing apparatus using the ultrasonic oscillating device.
In a conventional ultrasonic washing apparatus, an ultrasonic oscillating device is directly attached to a cleaning tank such as on the bottom thereof as an oscillating source, and an ultrasonic oscillator is connected to the ultrasonic oscillating device through a cable in order to drive the same.
Such an ultrasonic oscillating device generally comprises a vibrating base plate or diaphragm made of stainless steel and an ultrasonic transducer attached to the vibrating base plate by using an adhesive. In general, the thickness of the vibrating base plate of the ultrasonic oscillating device is determined to be a very small value, for example approximately 1/100 at most with reference to the wavelength of the oscillating frequency of the vibrating base plate oscillating by means of a drive oscillating signal output from the ultrasonic oscillator. For instance, -2 mm in thickness for the vibrating base plate is employed in an ultrasonic washing apparatus frequently using a drive ultrasonic wave having a 28 kHz wavelength.
Further, in general it is necessary to determine a length of one side of the vibrating base plate having a quadrilateral shape such as a square or a rectangle, to be at most a quarter-wavelength of a resonance frequency of the vibrating base plate, in order to effectively resonate the vibrating base plate.
In the conventional ultrasonic washing apparatus, a driving oscillating signal having a monofrequency is fed to the ultrasonic transducer of the ultrasonic oscillating device from the ultrasonic oscillator. The ultrasonic oscillating device emits an ultrasonic wave into a cleaning liquid in the cleaning tank, thereby washing and cleaning an object to be washed in the cleaning liquid of the tank.
However, when the frequency of the drive oscillating signal is determined to be at least 100 kHz, then the vibrating base plate must be extremely thinned. For instance, when the frequency of the drive oscillating signal is determined to be one MHz, then the thickness of the vibrating base plate is at most 0.1 mm. The thinning of the thickness of the vibrating base plate causes a drop in mechanical strength of the vibrating base plate of the ultrasonic oscillating device. In other words, when the ultrasonic oscillating device is mounted onto the bottom of the cleaning tank, the vibrating base plate is deformed by the pressure of the cleaning liquid. Thus, the ultrasonic transducer attached to the vibrating base plate peels off, or in the worst case., the vibrating base plate breaks down. Furthermore, when the thickness of the vibrating base plate is thinned, the vibrating base plate is liable to be deformed when sticking the ultrasonic transducer onto the vibrating base plate. Hence, the sticking operation becomes difficult and troublesome.
Reinforcing the vibrating base plate in order to increase the mechanical strength or to diminish the area of the vibrating base plate has naturally been considered, however new problems arise. For instance, irregular oscillation occurs between the reinforced part and the other part of the base plate, or the oscillating energy propagated to the cleaning liquid is reduced.
Furthermore, when the ultrasonic oscillating device is driven by the ultrasonic wave having a high-frequency of at least 100 kHz, it is not practical to design the length of one side of the quadrilateral vibrating base plate to less than a quarter-wavelength of the resonance frequency of the vibrating base plate, which is too small.
Then, when the length of one side of the quadrilateral vibrating base plate is normally designed to be at least a quarter-wavelength of the resonance frequency of the vibrating base plate, the resonant oscillation is largely damped, especially in the case of a thick vibrating base plate. Accordingly, the effective ultrasonic oscillation of the vibrating base plate cannot be attained.
Moreover, in the conventional ultrasonic washing apparatus, since the ultrasonic oscillating device is driven by the monofrequency drive oscillating signal, a variety of contamination of items to be washed cannot be properly treated. Thus the range of use of the apparatus is limited.